Uses addCursorRect:cursor: as discussed.. well, too long ago. Anyhow,
this will effectively hide the cursor while it is inside a window with
GLFW_CURSOR_HIDDEN or GLFW_CURSOR_CAPTURED enabled. This shouldn't mess
up cursor retain counts either, unlike previous uses of the hide/unhide
methods on NSCursor.
It does allocate a small, single-pixel image for an invisible cursor,
as well as the cursor itself, but that shouldn't be too much trouble.